The deciding factor in the selection of a d.c. motor for a particular application is its ___________

Question

The deciding factor in the selection of a d.c. motor for a particular application is its ___________ characteristic.

Examoogle · Accepted Answer

Speed-Torque

Explanation: The Speed-Torque characteristic is the primary deciding factor for a DC motor because it defines the mechanical capability of the motor to drive a specific load across its operational range. It demonstrates how the motor's shaft speed responds to the varying mechanical load (torque) required by the application.

Working Principle: In DC motors, the speed ($N$) and torque ($T$) are inherently linked through the armature current ($I_a$) and flux ($Phi$). Specifically, $T \propto \Phi I_a$ and $N \propto \frac{V - I_a R_a}{\Phi}$. Because every industrial load has a distinct torque-speed profile (e.g., constant torque, centrifugal, or constant power), selecting a motor requires matching its inherent $N-T$ curve to the load's demand curve to ensure stable operation and optimal efficiency.

Key Formulas: $T_a = K_a \Phi I_a$ — Electromagnetic torque relation; $N = \frac{V - I_a R_a}{K_b \Phi}$ — Speed relation for a DC motor

Key Points: Load requirements dictate the motor selection.; Shunt motors provide constant speed characteristics.; Series motors are preferred for high starting torque applications.; Compound motors offer flexible characteristics based on field winding configuration.

Advantages: Predictable performance under various load conditions; Ensures operational stability and prevents motor stalling

Disadvantages: Overlooking speed regulation can lead to thermal damage; Mismatching leads to inefficiency in energy conversion

Applications: Electric traction; Rolling mills; Centrifugal pumps; Hoists and cranes

Additional Info: The Speed-Torque curve is often referred to as the 'mechanical characteristic' of the motor.. Option B (Speed-Armature current) describes the internal electrical performance, while Option C (Torque-Armature current) relates to the electrical torque production, but neither fully captures the external load compatibility as effectively as the Speed-Torque curve.

Answer: A is correct — The Speed-Torque characteristic directly indicates the motor's ability to maintain a required speed under a specific mechanical load.

Exam Tip: Always remember: Speed-Torque relates to the load, while Torque-Armature current relates to the internal electromagnetic force generation.

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